C4 (Controlled Collapse Chip Connection) is an advanced microelectronic packaging and chip interconnect technique which is currently used in the semiconductor packaging industry. C4 technology is also known and referred to in the art as Flip Chip and solder bump technology.
The basic premise of C4 is to connect chips to ceramic packages by means of solder balls between two surfaces of the units. These tiny balls of electrically conductive solder bridge the gaps between respective pairs of metal pads of the units being connected. C4 provides a very high density electrical interconnection. Unlike other interconnection techniques, which make connections around the perimeter of a chip, C4 allows one or more surfaces of a chip or package to be packed with pads.
As the number of circuits on a chip increases, so does the number of connections needed. C4, which allows more connections in a smaller space than any other packaging technique, is increasingly important commercially.
One method of forming C4s uses an evaporation process wherein solder metal is evaporated through a metal mask in a vacuum chamber. This method, however, is typically less than 5% efficient; more than 95% of the evaporated metal ends up on the evaporator wall and on the metal mask.
An alternative technique described by Datta et al., J. Electrochem. Soc., 142, 3779 (1995) is the electrochemical fabrication of C4s. In this electrochemical fabrication of C4s, a continuous seed layer is required to provide the electrical path for through-mask electrode deposition of PbSn C4s. The seed layers are deposited on a wafer by vacuum deposition techniques, such as sputtering. A layer of photoresist is then applied and patterned to create vias. The solder is then electroplated. For high-end applications, the solder comprises a lead-tin alloy, with a lead content of about 95-97% (heretofore "97/3 PbSn" or "97/3"). After electroplating, the photoresist is stripped and the seed layer between the C4 pads is removed by etching, such that the remaining seed layer under the C4s acts as the ball limiting metallurgy (BLM) for solder pads.
Etching of seed layers is one of the critical processing steps in the electrochemical fabrication of C4s. The seed layers used in the fabrication of 97/3 PbSn C4s consist of Cu as the solderable layer, phased CrCu as the glue layer, and a TiW alloy typically containing about 10% Ti as the adhesion layer. The seed layers between the C4 pads have to be completely removed in order to eliminate electrical contact between C4s while the remaining seed layers under the C4s act as the ball limiting metallurgy (BLM) for solder pads. The size of the seed layers that remains under the C4s is very critical to obtaining mechanically robust C4s. It is essential to maintain large sized TiW pads under the C4s which can sustain the stresses in the flowed C4s. The seed layer etching consists of two steps: (i) electroetching to remove Cu and phased CrCu with minimum undercut of the layers, and (ii) chemical etching to remove the TiW layer.
U.S. Pat. No. 5,462,638 to Datta, et al. provides a chemical etching process based on hydrogen peroxide to selectively remove TiW in the presence of PbSn, CrCu, Cu and Al. The TiW etching bath disclosed in the '638 patent consists of a mixture of the following components: (a) hydrogen peroxide acting as the etchant; (b) potassium sulfate (or another soluble sulfate salt) acting as a passivating agent that forms protective layers over the PbSn C4s; (c) K.sub.2 EDTA (or another soluble EDTA salt yielding a solution pH less than 7) acting as a stabilizer for hydrogen peroxide, a buffer, and a complexant for the etched products.
The bath is operated at about 50.degree. C. An end-point detection method permits one to stop etching at a point that corresponds to complete removal of TiW between C4s, while providing a minimum undercut. Etch rate, undercut, and bath stability are some of the criteria that determine the etching process performance.
Both hydrogen peroxide and EDTA in the TiW etching bath degrade with time, leading to pH changes and degradation of etching performance. Accumulation of etched metallic ions in the bath also leads to degradation of the bath components.
The monitoring of TiW etching bath is complicated by the presence of high concentrations of hydrogen peroxide and of potassium sulfate, as well as by the gradual degradation of EDTA. The presence of peroxide precludes the use of common ion-selective electrodes; the high sulfate concentration complicates ion chromatography and several other techniques; and the decomposition products of EDTA are not known or easily determined. In addition, the analytical techniques have to be easy, fast and inexpensive to implement in a manufacturing environment.
In view of the drawbacks mentioned hereinabove, careful monitoring of the individual bath components of a metal etchant solution of the type mentioned above is essential to develop a robust TiW etching process for C4 fabrication that is capable of providing a tight control of TiW undercutting. Moreover, analytical methods are needed for monitoring the individual components of a metal etchant solution that are easy to implement and provide reliable data regarding the individual components of the metal etchant solution. The data found by these analytical methods can be used to determine when the etchant solution needs to be replenished or discarded.